The present invention relates broadly to a signal detector apparatus, and in particular to a hand off integrator apparatus for signal detection.
In a radar system, the video signal which is the result of target-reflected echoes coaprises a series of pulses which occur periodically at the repetition frequency of the radar system. Random signals which are the result of the combined effects of ground or sea clutter and noise signals that are generated within the receiver of the radar system are superimposed on the series of target-reflected echo pulses. This random clutter signal causes the real target signal to be masked, thereby making it difficult, if not impossible to detect small targets.
In order to improve the signal-to-noise and the signal-to-clutter ratio of a radar system, a type of integrator circuit which is commonly known as a sweep integrator, has been developed. Basically, a sweep integrator consists of a memory circuit, such as an ultrasonic delay line, with a controlled feed-back circuit coupling the output of the memory circuit to the input in order to form a recirculating signal loop. The delay time of the memory circuit must be chosen to be equal to the radar pulse repetition period. Radar video signals are introduced into the recirculating signal loop in which they are combined with previously introduced radar video signals. An output signal which is proportional to the amplitude of a selected characteristic of the combined signal is derived from the recirculating loop. Since the integrator stores and integrates all the echoes that are received from a particular radar sweep, it is called a sweep integrator.
Sweep integrators operate on the principle that the frequency distribution of a recurrent series of pulses is a line spectrum with the lines located at harmonics of the pulse repetition frequency. In comparison, the spectrum of random noise and certain types of clutter signals is more or less uniformly distributed over the entire video passband. The sweep integrator is generally arranged such that periodic signals which represent echoes from targets add linearly in the recirculating loop while aperiodic signals, which are the result of clutter and thermal noise, add in the more gradual, root means square manner. As a result, the signal-to-noise and/or clutter ratio on the recirculating loop is greater than the corresponding ratio of the non-integrated signal. The improvement of the signal-to-noise and/or clutter ratio is a function of the number of successive pulse series which are stored as a sum in the recirculating loop. In a typical application of the sweep integrator it may be desirable to store with appreciable strength the sum of as many as 50 to 100 successive series within the recirculating loop. In order to achieve optimum storage of 50 to 100 successive series with exponential memory (based on the criterion of peak signal to R.M.S. noise) the feedback factor must be equal to 0.98 or 0.99. The feedback factor will approach the value 1 as the number of series to be stored increases but must average less than 1 if the system is to be stable or non-oscillatory.